AP5 (also known as APV, (2R)-amino-5-phosphonovaleric acid, or (2R)-amino-5-phosphonopentanoate) is a chemical compound used as a biochemical tool to study various cellular processes.

AP5 blocks the cellular analog of classical conditioning in the sea slug Aplysia californica, and has similar effects on Aplysia long-term potentiation (LTP), since NMDA receptors are required for both.

[2] It is sometimes used in conjunction with the calcium chelator BAPTA to determine whether NMDARs are required for a particular cellular process.

[3] In general, AP5 is very fast-acting within in vitro preparations, and can block NMDA receptor action at a reasonably small concentration.

The active isomer of AP5 is considered to be the D configuration, although many preparations are available as a racemic mixture of D- and L-isomers.